题记

最近做项目遇到的问题，程序跑了多个process，每个process都是相互独立的，为了解耦，类似于微服务的架构，我们要求系统可以detach 到 主线程跑飞，死循环等其他bug 问题，最初的设计方案是：每个process 都会给每一个monitor的process 去发送keep alive 消息，由monitor去收集每个module的keep alive消息，然后去判断是否process 跑飞等情况。但是这种方案，由于需要多一个monitor模块，在本来内存有限的嵌入式设备上，有点得不偿失，后来就想能否有linux 系统内部的 实现可以达到我们的要求，也就是 SIGALRM

1. Signal & Semaphore 区别

Signal: 是通过软中断信号通知进程发生了异步事件。进程之间可以通过系统调用kill 发送软中断信号，内核也可以因为内部事件而给进程发送信号，通知进程发生了某个事件。

Semaphore: 信号量是用来操作系统进程间同步访问共享资源。信号量在创建时需要设置一个初始值，表示同时可以有几个任务可以访问该信号量保护的共享资源，初始值为1就变成互斥锁（Mutex），即同时只能有一个任务可以访问信号量保护的共享资源。

2. SIGALRM 以及python code 实现

SIGALRM是在定时器终止时发送给进程的信号，在进行阻塞式系统调用时，为避免进程陷入无限的等待，可以为阻塞式系统调用设置定时器。 #include <unistd.h> unsigned int alarm(unsigned int seconds);

在alarm成功调用后，开始计时，超过该事件将触发SIGALARM信号，然后会调到handler 执行。如下 是python的例子，

import signal,time,sys,thread,traceback class Example: def __init__(self): self.handler_counter = 0 self.retry_counter = 3 pass def timout_handler(self, signum, frame): ''' timeout handler when failed to send signal alarm there is a retry to make sure main thread hung ''' self.handler_counter += 1 print "call timeout_handler counter: " + str(self.handler_counter) if self.handler_counter == self.retry_counter: print("Have retry %s, exit process", self.retry_counter) traceback.print_stack(frame) # print traceback sys.exit() def monitor_alive(self, threadName, delay): ''' monitor alive to send alarm message every (delay + 1) second, if after (delay + 1) doesn't receive response from kernel, will interrupt timout_handler ''' count = 0 while True: time.sleep(delay) signal.alarm(delay + 1) print "sign_time count " + str(count) # below if logic to mock 3 time timeout if count == 2: time.sleep(delay) if count == 4: time.sleep(delay) if count == 6: time.sleep(delay) count += 1 print "%s: %s" % (threadName, time.ctime(time.time())) if __name__ == '__main__': example = Example() # register handler # only could set signal handler in main thread # https://stackoverflow.com/questions/44151888/why-only-main-thread-can-set-signal-handler-in-python signal.signal(signal.SIGALRM, example.timout_handler) thread.start_new_thread(example.monitor_alive, ("Thread-1", 2,)) while True: time.sleep(2) print('main thread ')

运行结果：

sign_time count 0 Thread-1: Sat Jun 9 10:52:43 2018 main thread sign_time count 1 Thread-1: Sat Jun 9 10:52:45 2018 main thread sign_time count 2 main thread Thread-1: Sat Jun 9 10:52:49 2018 main thread call timeout_handler counter: 1 main thread sign_time count 3 Thread-1: Sat Jun 9 10:52:51 2018 main thread sign_time count 4 main thread Thread-1: Sat Jun 9 10:52:55 2018 main thread call timeout_handler counter: 2 main thread sign_time count 5 Thread-1: Sat Jun 9 10:52:57 2018 main thread sign_time count 6 main thread Thread-1: Sat Jun 9 10:53:01 2018 main thread call timeout_handler counter: 3 ('Have retry %s, exit process', 3) File "/home/odl/sereno/tests/singal.py", line 52, in <module> time.sleep(2)